1. Field of the Invention
The present invention relates to a control signal generating circuit of an oscillating signal generating device, and more particularly to a control signal generating circuit capable of using any voltage level between a supply voltage source and a ground voltage to control the oscillating signal generating device.
2. Description of the Prior Art
Advanced semiconductor manufacturing processes for manufacturing integrated circuits can reduce the size of the integrated circuits. When an integrated circuit has a reduced size, the operating voltage range of the integrated circuit is smaller, meaning the voltage range of a control signal of the integrated circuit is also smaller. When the controlling voltage range of a voltage-controlled oscillator becomes smaller, the gain (Kvco) of the voltage-controlled oscillator should be larger in order to generate an oscillating output signal having a predetermined frequency range. As a result, the voltage-controlled oscillator may become very sensitive to the control voltage, which induces unwanted jitter in the oscillating output signal as a consequence. When the above voltage-controlled oscillator is applied in a phase-locked loop, the loop filter of the phase-locked loop needs to be able to filter out the induced jitter, which is only possible if the filtering capacitor of the loop filter is large enough. The cost of the phase-locked loop becomes higher if a large capacitor is required, however. Therefore, how to reduce the control voltage range of a control signal of a voltage-controlled oscillator while reducing the cost of the voltage-controlled oscillator has became an urgent problem in this field.